1. Field of the Invention
The present invention relates to a method of measuring an amount of missed tissue at chest wall side by placing a phantom on a support table for breast radiography and to a phantom used in this method.
2. Description of the Related Art
Apparatuses for radiographing and reading radiographic images are known in the field of medicine. In an apparatus of this kind, radiation emitted from a radiation source and carrying a radiographic image of a subject after passing the subject is irradiated on a radiographic image conversion panel to record the radiographic image of the subject on the panel, and the radiographic image is obtained by reading the radiographic image conversion panel. As such a radiographic image conversion panel, there is a known panel that stores a portion of energy of radiation emitted thereon and emits light with intensities corresponding to the stored radiation energy upon exposure to stimulating ray such as visible light. There is another known panel that temporarily stores electric charges generated by exposure to radiation in an electric charge storage unit of a solid state detection device and outputs the stored charge after converting the charges into an image signal.
In addition, a breast radiography apparatus that radiographs and reads a radiographic image of a breast is known as a type of radiography apparatus (see Japanese Unexamined Patent Publication No. 2004-154409). In the breast radiography apparatus, a breast of a subject is fixed on a support table having a radiographic image conversion panel installed therein, and radiation is emitted from a radiation source above the support table, to record the radiographic image of the breast in the panel.
The support table has a surface that is in contact with a chest wall of the subject (hereinafter referred to as the chest wall side) at the time of radiography. The breast radiography apparatus is set to irradiate the radiation from the radiation source as close as possible to the chest wall but not on the chest wall. In other words, the positional relationship between the radiation source and the support table is adjusted so as to cause the radiation from the radiation source to be emitted as close as possible to the chest wall side but not beyond the chest wall side. The radiographic image of the breast including an area close to the chest wall can therefore be obtained by this adjustment.
The positional relationship between the support table and the radiation source is basically fixed. However, this relationship may change due to various reasons, and a radiation field on the support table, that is, a region of radiography, needs to be measured occasionally. As a method of measuring deviations in the region of radiography, a method using a phantom is known. In this method, the distance between a chest wall side of a support table and an edge of a radiography region, that is, an amount of missed tissue at chest wall side, is measured by use of a phantom.
As a phantom used in this method, a phantom having iron balls having diameters of 2 mm aligned in one direction in a block is known (see Ryuji Suzuki et al., “Academic Publication of Radiation Medicine and Technology 14-3; Mammography Accuracy Management Manual (Revision 3)”, Publication Committee of Japanese Society of Radiological Technology, Dec. 28, 2004, pp. 78-80). The iron balls are aligned so as to be perpendicular to a chest wall side when the phantom is placed on a support table.
The amount of missed tissue at chest wall side can be obtained by radiographing the phantom in a state where a reference surface of the phantom is in contact with a chest wall side of a support table. In other words, by counting how many iron balls aligned in one line have been radiographed or not radiographed in a radiographic image, the amount of missed tissue at chest wall side representing the distance between the chest wall side to an edge of a radiography region can be found.
In measurement of an amount of missed tissue at chest wall side by using the phantom having the iron balls aligned in one direction, an image representing the iron balls radiographed and displayed by a breast radiography apparatus is visually evaluated. However, measurement accuracy is not sufficient only by viewing in this manner, and there is demand for measurement of an amount of missed tissue at chest wall side with higher accuracy.